Bluetooth: tester: Fix copying attribute to the server_buf

BT_GATT_INCLUDE_SERVICE user_data shall point to the service declaration
of service to be included. So user_data_len shall be set to 0 to avoid
copying the contents of bt_gatt_attr to the server_buf, and just use
pointer to the actual attribute.

Change-Id: Id770daeb6d64644c51f2bc5d3962dd80363ec8c3
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>